New Member with a Graphics Problem

[Jbbnbsmith]

Hi everyone. I've been using my Atari STf 1040 (upgraded to 2 MB) since about 1986. Last summer when I tried to turn it on I experienced a graphics/video issue.

When I first power up all I see is the top menu bar text repeated on the screen.

After about 20 minutes the screen starts flickering and you can see the green background and the folder icons, but they are scrambled.

After an hour or so the screen settles down and looks almost normal except that sections of rows are shifted.

(I'll post a pic when I make the image smaller)

I did some research and most things I read suggested it was the power supply. Although it looked like I was getting proper voltage, I just put a new Mean Well RD35a in my machine, and I am getting a very consistent 5.16 V (and 12 V).

However, I am still getting the exact same issue. Everything is completely reproducible including the times.

I've tried pulling and replacing all the chips but that did not change anything.

I'm going to try a more detailed cleaning of the MMU and GLUE chips (I found a link for that here), but given the transient nature of the problem I am wondering if it could be a capacitor on the motherboard.

I know a bit about circuits and have some crude soldering skills but I'm at a loss as to what may be the issue. Is it worth replacing some capacitors or do you think it is more likely something else.

By the way, I did try searching the forums but I must have been doing it wrong because I kept getting zero results. So if this topic has been covered pleased post the link.

Thanks in advance for the help! I'd really like to get this old machine running again.

[PhilC]

@Jbbnbsmith try and post a photo of each change of phase but to me it sounds capacitor related


Btw the forum will resize the image automatically.

[Jbbnbsmith]

Thanks for the approval!

I'm going to attached a few pics from my phone. I apologize if the resolution is too high.

This is where it sort of ends. There is minimal flickering, but it still does every once in a while.

15950929067451443134062.jpg (366 KiB) Viewed 2432 times

This is how is starts when you first turn it on after being off for a while.

IMG_20200717_234158554_HDR.jpg (310.98 KiB) Viewed 2432 times

It stays like that for about 20 minutes and then starts noticably flickers regularly and slowly becomes the first pic after maybe an hour.

If after an hour I turn it off and back on it still looks like the first image. Just to rule out the monitor I tried turning off the computer for a while and left the monitor on, but doing that the sequence started all over. So it's definitely (no surprise) the computer.

If the issue is a capacitor, is there a specific one that is the culprit? I don't have a meter to check capacitance. Would checking ohms be useful?

By the way, as I'm typing this the screen is flickering again and I occasionally see the proper display in brief flashes.

[terriblefire]

This *could* be clock gitter to the shifter....? So possibly caps

[exxos]

Have you cleaned the shifter, MMU, GLUE pins with IPA ? Also you may need to resolder them.

[Jbbnbsmith]

Have you cleaned the shifter, MMU, GLUE pins with IPA ? Also you may need to resolder them.

Not yet, I only pulled them and reseated them. I'm going to try cleaning them good and I'll take a look at the solder connections. I think they looked okay, but I'll dig deeper.